1. Field of the Invention
The present invention relates to a method of manufacturing a radiation-resistant optical fiber and, more particularly, to a method of manufacturing an optical fiber with a high radiation resistance which can be used as, e.g., an optical fiber cable in nuclear associated plants or nuclear reprocessing plants in which exposure to a high radiation dose is expected.
2. Description of the Related Art
A radiation-resistant optical fiber must be used as an optical cable for use in environments in which a radiation dose is high. An optical fiber having a pure silica core has been conventionally used as such a radiation-resistant optical fiber.
FIG. 1 is a sectional view showing the structure of an optical fiber having a pure silica core. Referring to FIG. 1, reference numeral 1 denotes a core consisting of pure silica; and 2, a cladding. A bare optical fiber is constituted by the core 1 and the cladding 2. The bare fiber is double-covered with a primary coating layer 3 consisting of a modified silicone resin and a secondary buffer coating layer 4 consisting of a plastic material such as nylon.
The optical fiber having a pure silica core has a higher radiation resistance than that of a normal optical fiber having a GeO.sub.2 -doped core.
Although the conventional pure silica core optical fiber has a high radiation resistance as described above, the following characteristic degradation occurs if an exposed radiation dose becomes 10.sup.6 R or more. That is, SiO.sub.2 in the optical fiber is destroyed by radiation to cause NBOHC (Non Bridging Oxygen Hole Center; Si--O.multidot.), and hydrogen gas reacts with this NBOHC (Si--O.multidot.) to produce an OH group. When the OH group in the fiber is increased in number, optical communication using a wavelength band of 1.3 .mu.m becomes impossible due to increase of optical absorption loss caused by the OH group.
Note that possible supply sources of the hydrogen gas which reacts with the NBOHC (Si--O.multidot.) to produce the OH group are hydrogen gas occluded in the optical fiber, or hydrogen gas generated when the primary and secondary buffer coating layers are decomposed by radiation.